Commercial products prepared in continuous webs are commonplace and include paper rolls, plastic films, and the like. Frequently, the web is surface treated in some manner. For instance, the web can be used as a substrate upon which various coatings are applied (eg. as with sticky tape or audio tape). Many continuous web products are calendered in order to compact an applied coating or even to compact the web itself (eg. paper). Calendering is typically done by running the web through a set of calendering rollers where the nip or gap between the rollers is set in accordance with the amount of compaction desired. Many techniques for processing webs appear in the art generally and many varied types of related apparatus have been developed for commercial use. An excellent reference on this subject is "Web Processing and Converting Technology and Equipment", edited by D. Satas, Van Nostrand Reinhold, 1984, wherein a detailed overview of the art is presented.
With the introduction of lithium ion batteries in the marketplace, a new specialty area pertaining to web processing has been created. Lithium ion batteries are a preferred rechargeable power source for many consumer electronics applications, particularly laptop computers and cellular phones, and such batteries have been available commercially since about 1991. Lithium ion batteries are characterized by a large energy density (Wh/L) and high operating voltage (typically above 31/2 volts).
Due to the relatively low ionic conductivity of the non-aqueous electrolytes employed in these batteries, very thin electrodes (circa 100 micrometers thick) are generally used in order to obtain reasonable discharge and recharge rate capability. These thin electrodes are typically made in continuous webs by coating a suitable current collector material with the appropriate active electrode material.
The conventional construction of commercial lithium ion batteries is described in many references, including Canadian patent application serial numbers 2,147,578 (filed Apr. 21, 1995) or 2,163,187 (filed Nov. 17, 1995). Two different lithium insertion compounds are used therein for the active cathode and anode materials that both have ample capacity for reversible lithium insertion but that have differing lithium insertion potential. At this time, a lithium transition metal oxide (eg. LiCoO.sub.2, LiNiO.sub.2, LiMn.sub.2 O.sub.4) is usually employed as the cathode material and a carbonaceous compound (eg. coke, graphite, hard disordered carbon) is usually used as the anode material. Various lithium salt and non-aqueous solvent combinations are used in the battery electrolyte.
Commercial products typically appear in cylindrical (eg. 4/3 A size) or prismatic (a rectangular parallelepiped shape) formats and typically contain a spiral winding inside, often referred to as a jelly roll. The jelly roll is a spiral winding of thin continuous web components including a cathode foil, an anode foil, and two microporous polyolefin sheets that act as separators. Both sides of the cathode and anode foils are coated with active electrode material.
Cathode foils for the jelly roll are prepared by applying a mixture of a suitable powdered (circa 10 micron particle size) cathode material (eg. LiCoO.sub.2), a binder, and a conductive dilutant onto a thin aluminum foil web (circa 10 micron thick). The aluminum foil web serves as a mechanical substrate or support for the active cathode powder and also as an electrical current collector in the assembled battery. Typically, the application method first involves dissolving the binder in a suitable liquid carrier. Then, a slurry is prepared using this solution plus the other powdered solid components. The slurry is then coated uniformly onto the substrate foil using a coating method suitable for accurately applying powder slurries. Afterwards, the carrier solvent is evaporated away. The amount of web substrate that ultimately winds up in the assembled battery is usually minimized in order to make the most use of the space available inside for active electrode material. In this way, the all important capacity of the assembled battery can be maximized. Thus, the thinnest web that can practically be handled is used. Generally, manufacturers coat both sides of the foil substrate in order to minimize the net thickness of substrate appearing in the assembled battery (ie. rather than use two single side coated foils back-to-back in the jelly roll winding).
After the coating is applied, the cathode foil is calendered to compact the porous powdered active coating. Again, this maximizes the amount of active electrode material that can be stuffed into a battery container and hence maximizes battery capacity. Calendering also can desirably improve electrical contact between the particles in the coating and can further improve adhesion between the particles and between the coating and the foil. The extent of the conventional calendering is typically limited by mechanical considerations rather than by battery performance considerations. Although greater compaction can be desirable from the battery design perspective, greater compaction can severely distort the web substrate such that it can no longer be handled.
Anode foils are prepared in a like manner except that a suitable anode powder (eg. a graphitic carbon) is used instead of the cathode material and thin copper foil is usually used instead of aluminum.
Because the metal foil webs serve as electrical current collectors in an assembled battery, some sort of electrical connection must be made to the webs. Typically, a flexible metal tab is welded to each respective metal foil web. However, in order to access a web, portions of the web must be exposed, either by cleaning off the coating or by leaving certain sections uncoated. The latter option is often preferred since it obviates the need for subsequent cleaning of the coated web. While segment coaters may not be able to coat quite as quickly as continuous coaters, battery assembly overall can be more efficient (and is certainly a cleaner process) if web cleaning operations can be avoided.
Segment coater devices have been developed recently with this purpose in mind. Such coaters can apply segment coatings on both sides of a metal foil web for use in lithium ion battery applications. With these segment coaters, not only can the thicknesses of the segment coatings be precisely controlled, but also the alignment and edges of the segment coatings can be precisely controlled such that small, aligned uncoated. sections can be reproducibly formed on the coated web for purposes of attaching tabs thereto. Canadian patent application serial number 2,093,898 of Moli energy and Japanese laid open patent application number 01-184069 of Sony both disclose coating apparatus suitable for this kind of segment coating.
As manufacturers attempt to achieve ever greater compaction of lithium ion battery electrodes, problems specific to segment coatings have been observed. At the discontinuities in the segment coating (or edges thereof), sudden transitions occur during the calendering process. Such transitions result in sudden loadings and unloadings of the calendering roller apparatus and of the web itself. This can result in machine vibration (known as `knocking`) and damage to the calender apparatus. Further, it can result in damage to the web. Tearing of the web, particularly at the trailing edges of segments, can occur during extreme calendering. And, even if tearing damage to the web is not immediately apparent, calendering damage may result in a weakening in the integrity of the foil (eg. embrittlement as a result of cold working) and lead to subsequent failure in an assembled battery.
At the edges of the segment coatings, the thickness of the coating can be slightly greater than that of the bulk coated segment. These `bumps` can further aggravate the aforementioned problems.
Thus, the use of conventional segment coating methods can unduly limit the amount of calendering that can otherwise be achieved on continuously coated webs. It is therefore desirable to develop means for minimizing the machine and web damage that are associated with segment coating in order to obtain greater levels of compaction.